In the field of paperboard food containers which are typically formed by compressing paperboard blanks between dies, there is a need, especially in the fast food industry, for effective sealing between a formed tray and its lid structure to maintain the temperature and moisture level of the contents of the container. This is accomplished by a locking means between a lid and the tray which can be easily engaged after the food is placed in the tray. After engagement, the food container should seal the contents from the environment, especially if the contents are hot, cold or spoilable. Additionally, the locking means should allow easy removal of the lid structure for access to the contents of the container.
Conventional embodiments of releasably interlocking means on paperboard containers includes a Peerless structure which is believed to be the subject of a patent application of another. The Peerless structure is of the clamshell variety and comprises a lower tray section having a slot near its upper ledge and a matingly formed lid structure having a tab which extends outwardly and into the slot during locking. While this releasably interlocking means is easily engaged and disengaged, the sealability of the container is inadequate. The poor sealing is due to an elevated ridge structure on the upper ledge at the slot means which prevents complete seating of the lid against the ledge of the lower tray. The elevated ridge structure is angled relative to the upper ledge at the slot, preventing the lid from contacting the upper ledge around the entire circumference of the tray. Additionally, the Peerless structure does not have a lid seating ledge near the slot, but merely an upwardly facing edge, of paperboard thickness, which is recessed downward from the lid seating ledge plane and therefore prevents sealing at the tab.
The need therefore exists for a releasably interlocking means on a food container which is easily engaged and disengaged, but most importantly accomplishes complete sealing of the contents of the container.